Researchers examine deformities in amphibian limbs

In 1995, a group of schoolchildren in the wetlands of rural Le Sueur, Minn., yelled out to their teacher.

“This one has no legs on one side and a bunch on the other!” one student said, according to a PBS transcript chronicling the discovery of the deformed frogs.

Most schoolchildren would yell to their teachers if they had encountered frogs with extra or missing limbs. After all, a young student would envision a frog with four limbs — no more and certainly no less.

A group of Yale researchers from the School of Forestry and Environmental Studies, the School of Medicine and the Department of Biology are collaborating in an inter-departmental project that has one common tie — the deformities of amphibian limbs in the wetlands of Vermont, which are the same as those found in the Minnesota marshes.

The team, led by ecology and evolutionary biology professor David Skelly, will soon be publishing its findings in the journal Environmental Health Perspectives. The study attempts to invalidate the previous theory that the deformities found in wetland frogs were a result of the parasite Ribeiroia. Skelly’s theory instead relies upon his belief that run off from agricultural and cultivated lawns may be causing these maladies in the Vermont amphibian population.

“We have a major deformity outbreak, and it is not necessarily associated with parasitism as previously thought,” Skelly said. “Chemical pollution makes more sense, and, when satellites are used to examine ground use, you can see that in all these areas, there is a prominent agricultural industry.”

Dr. Peter Rabinowitz, professor of occupational and environmental medicine, said ecologists fall into one of three camps when describing the origins of these amphibian deformities — parasites, toxins and UV-light.

The scientists are still unclear as to what chemicals may be causing the deformities, although their fingers are pointing at pesticides.

“Quite frankly, we don’t really know what is causing this, if parasites are not,” Skelly said. “There certainly is evidence that pesticides have caused limb deformities in other species in the past.”

The evidence for Skelly’s theory comes largely from the fact that amphibians in Vermont have too few limbs, rather than an overabundance of them. Had parasites been the answer to the limb mystery, there would be a numerous frogs with duplicate limbs, Skelly said.

The way the parasite works, he said, is that it infests a developing limb bud, effectively cutting off communication between the limb and the rest of the body so that the amphibian’s developmental system is not signaled that a limb is forming. The result is a repeated signal, and a new limb forms in addition to the one to which the parasite attached itself.

Since the frogs of New England are losing limbs, the researchers posit that this cannot be the answer. The team of researchers feel something in the environment, possibly within reach of humans, is causing this abnormality.

“Are the frogs telling us something?” Rabinowitz said. “The question is that ever since those kids found the deformed frogs in Minnesota, is there a chance that human health will be affected by it? I think we have to pay close attention.”

Rabinowitz said he believes there is too large a gap between animal medicine and human medicine, when really the two may go hand-in-hand and tell us something about what we can be expecting.

While the mystery remains unsolved, future research is heading into an area of great intrigue, Rabinowitz said. The team will attempt to determine whether or not what is causing the deformities in the frogs is able to cause deformities in humans exposed to the same agricultural pesticides and chemicals, he said.

“We have to figure out if this stuff is in our drinking-water supply,” Rabinowitz said. “Frogs can be the most susceptible to this particular toxin, but we have to find out the environmental hazard that this poses for us.”

Forestry and Environmental School Dean Gus Speth said Rabinowitz and Skelly’s research could be a giant leap for human health, not just that of frogs.

“Skelly is an accomplished and careful scientist, and a real leader in our school,” Speth said. “We all admire his judgement.”

But what about our leaping, green friends here in the Elm City?

“Somehow, the frogs in our local wetlands are doing well and have been relatively unharmed when compared to their neighbors in Vermont,” Skelly said.